bpf: Report which member makes a kfunc return type unsupported

A kfunc that returns a struct by value may only return scalars, and the
message that rejects one names the type but not the member at fault:

  kernel function bpf_kfunc_call_test_ret_ptr returns STRUCT
  prog_test_ret_ptr that is not composed of scalars

Record the member that made the walk fail and name it. What is recorded
is a path, not a single member, because the walk descends up to 4 levels.

For

  struct outer { struct inner { void *p; } in; __u64 tag; };

naming 'p' alone would send the reader looking for a member struct outer
does not have, so the actual message reads "member 'in.p' has type PTR".

For

  struct s { void *p[2]; };

naming 'p' alone would blame the member, when it is the element type that
was rejected, so the actual message reads "member 'p[]' has type PTR". A
zero-length array has no element type to name and is rejected for its
length instead, so it reports "member 'p' is a zero-length array".

The detailed diagnostics for this failure:

  Verification failed: Program Structure: Unsupported kernel function
  return type

  Reason:
    bpf_kfunc_call_test_ret_ptr() returns STRUCT prog_test_ret_ptr by
    value. Its member 'p' is PTR, not a scalar. Only kfuncs returning
    scalar values, or structures composed of scalar values are
    supported.
  ...
  Suggestion:
    Call a kernel function that returns only scalars by value.

A type nested deeper than the walk descends has no single member to
blame, so that case reports the depth instead:

  Reason:
    bpf_kfunc_call_test_ret_deep() returns STRUCT prog_test_ret_deep by
    value. It nests structs more than 4 levels deep. ...

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260829061545.1695736-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

0 files changed, 0 insertions, 0 deletions